Electrochromic modules for use as light filters, displays, dazzle-free rear view mirrors and the like are known. These involve reversible electrochemical oxidation and reduction of redox-active materials such as tungsten oxide, viologen or various polymers such as polythiophene, polyethylenedioxythiophene (PEDOT) derivatives, polyaniline inter alia, which changes the color thereof. Even though the various known electrochromic systems work quite well in individual cases, there are also a number of disadvantages. The electrochromic materials such as bipyridinium compounds (viologens) can be switched between three redox forms, reversibly from the dication to the radical cation and irreversibly to the uncharged form. In this case, the pimerization of the radical cations (formation of a π-complex through the π-electron planes) causes an altered absorption spectrum and has an adverse effect on the color contrast and the lifetime of the EC systems.
Stabilization materials are required, such as metallocenes and metallocene derivatives (DE 102007037619A1, US 2009/0259042A1, DE 102008024260B4) and also other compounds known, for example, from EP 1288275A2 and DE 102006061987, which, by guaranteeing a reversible anodic component reaction, ensure an improved lifetime of the cathode-switching electrochromic formulation (preferably 4,4′-bipyridinium salts) with regard to long-term contrast stability. Here, however, there are likewise problems with regards to color contrast and lifetime. In long-term studies, formation of metallocenium cations becomes perceptible through formation of a yellow-brown layer at the anode. Moreover, the addition of metallocenes to an electrochromically active formulation leads to separation processes which have been uncontrolled to date, for example the deposition of ferrocene aggregates.
Most of the electrochromic materials of significance for applications are switchable only between two colors: viologens (colorlessblue/violet), tungsten oxide (WO3) (light blueblue), poly-3-hexylthiophene (violetblue), polyethylenedioxythiophene derivatives, such as polyethylenedioxythiophene-polystyrene sulfonate (PEDOT-PSS) (light bluedark blue). It is thus possible to implement only two-color filters or only monochrome displays.
Moreover, many electrochromic materials, for example WO3 or PEDOT-PSS, are merely pseudo-colorless in thin layers, and so they are only of limited suitability for the applications where the colorless state is required within a broad wavelength range (500-nm).
Numerous studies have been conducted to date with regard to organic materials using the electrochromic effect. The great advantage of the electrochromic polymers and the controllable multichromicity thereof through modification of the chemical structure, and the inexpensive production of arbitrarily thin and large-area layers both on glass or metal substrates and on flexible films and textiles.
Known polymers suitable for electrochromic applications are polythiophenes, polypyrrole, polyphenylenevinylenes and polyaniline. However, these electrochromic conductive polymers have a tendency to alterations under air, especially with regard to the electrical properties and electrochemical stability thereof, and as a result have only a short lifetime. It is therefore important to encapsulate the EC modules and to protect them from outside influences. In this context, the necessary rigid encapsulation impairs the flexibility. Moreover, such polymers have a low glass transition temperature, and polypyrrole and polyaniline, for example, have a poor solubility, and so they are processable with difficulty. These disadvantages constitute serious hindrances to the practical use thereof.
Particular polymers having di- or triarylamine units are known as hole conductors, electroluminescent materials and light-emitting materials, and also multicolored electrochromes (W. Holzer et al., Optical Materials 15, 2000, 225-235).
Examples of the use of diphenylamine and derivatives thereof as electrochromic material or in combination with anthraquinones are described in U.S. Pat. No. 4,752,119. It has been proposed that a solution of a diphenylamine and a conductive salt in a chemically stable organic solvent (preferably propylene carbonate) between two electrodes be used. A TiO2 scattering layer was applied to an electrode, in order to better perceive the color change on the white background. As a result of the application of a voltage of 1.0 V to 1.5 V, the solution takes on a green color. If the voltage is increased to 2.2 V, a blue-green color forms in the solution. If the voltage is switched off, the system returns to the colorless ground state via diffusion. After 106 switching cycles, only relatively small electrochromic deteriorations in the cell were registered. However, such systems comprising liquid media are problematic in terms of the operating temperatures and lifetime; therefore, they have to be hermetically encapsulated.
The invention according to DE 3717917 relates to a novel polymer which consists of repeat units of N,N,N′,N′-tetraphenyl-p-phenylenediamine and has electrochromic properties. The polymer is soluble in organic solvents and only becomes insoluble once it has been doped with an electron acceptor and then dedoped. This polymer film shows a yellow color in the potential region of 0.3 V (with respect to Ag/AgCl), a green color in the oxidized state of the first stage at 0.85 V, and a dark blue color in the oxidized state of the second stage at 1.2 V. An electrochromic display was produced through the following steps: a transparent glass plate was subjected to vapor deposition of an insulation film of MgF2 (80 nm) outside the display region, then coated with the abovementioned polymer from a chloroform solution (200 nm), subsequently doped with iodine at 100° C. and then dedoped under high vacuum. On another glass plate coated with a graphite fiber layer, a Prussian blue film (300 nm) was electrolytically deposited. Between the two glass sheets was disposed a porous background panel made from alumina, and the two electrodes were sealed. The electrolyte used was 1 mol/l solution of LiClO4 in propylene carbonate. This electrochromic display was switched repeatedly up to 105 times by applying a coloring voltage of 8 V and a lightening voltage of −8 V. In the course of this, only a small change in the amount of charge was determined in the oxidation reaction compared to the starting value. The production of the electrochromic display is a multistage operation, combined with various different technological operations (doping with iodine at 100° C., dedoping under high vacuum, electrolytic deposition of Prussian blue), which leads to increased technical complexity and investment. Furthermore, the coloring and lightening voltages off ±8 V are very high compared to conventional EC cells and are economically disadvantageous.
DE 3615379 A1 describes a dazzle-free mirror. The first electrochromic layer is formed from a conjugated polymer such as a substituted or unsubstituted triphenylamine, and the other EC layer is a transition metal oxide, such as WO3. In the process described, a film is applied to the electrode from suitable triphenylamine monomer or polymer solutions using a coating process and is subsequently polymerized or crosslinked by means of oxidizing agents, such as iodine, antimony pentafluoride, arsenic pentafluoride or iron oxide. A further means of film formation is an electrolytic polymerization from monomer solution. For example, such a mirror consists of 4,4′-dichlorotriphenylamine polymer and WO3 EC layers with an electrolyte solution of LiClO4 in propylene carbonate with 3% by weight of water. The reflection of the mirror in the ground state is about 70%. In the case of application of a voltage of about 1.45 V, the mirror turns dark blue within about 4 s, and so the reflection is lowered to about 10%. A voltage of about −0.35 V led to decoloring of the mirror. The subsequent coloring (1.1 V, 15 s) and decoloring (−0.4 V, 90 s) were stably reproducible even after 30 000 repetitions. The in situ polymerization or crosslinking of the coating film has the disadvantage that residues of the oxidizing agent in the film can lead to unwanted side reactions in the case of repeated oxidation and reduction, and as a result to an unsatisfactory lifetime of the device. Equally, it gives an additional methodological step for practical use.
Electron-rich triphenylamines have a tendency to be oxidized in the presence of oxygen and light to form an unstable radical cation, which dimerizes further to a tetraphenylbenzidine. This oxidation leads both to yellowing of the polymer layers and to a limitation in the lifetime of the EC elements. By exchange of a group in the para-phenyl position, the dimerization reaction can be significantly reduced. However, it has been published recently that the conjugated homopolymer poly(4-methoxytriphenylamine) has only a moderately stable EC effect up to about 50 cycles (G.-S. Liou et al., Journal of Polymer Science: Part A: Polymer Chemistry, (2007), V. 45, 3292-3302).
Preparation and basic electrochemical properties of polymers having aryl-substituted arylenediamine polymers are present in DE 19832943. It has been found that the electrooxidation of a solution of a 3,3′-substituted triphenyldiamine dimer polymer (TPD polymer) reversibly gives rise to a blue color.
It is desirable to use TPD and tetraarylbenzidine polymers as electrochromic materials in an electrochromic module with suitable electrolyte and a suitable ion-storage or charge-compensating layer, which ensures the performance of redox reactions with favorable cyclic periodicity and hence a stable EC effect.